About this tool
This due date calculator estimates when a baby is likely to arrive and where a pregnancy stands today. Start from whichever date you know best: the first day of the last menstrual period (with an adjustment for cycles shorter or longer than 28 days), the date of conception or ovulation, the date of an IVF embryo transfer (day-3 or day-5), or a due date already given by a clinician, which the calculator works backward from. It then shows the estimated due date with its weekday, the current gestational age in weeks and days, the trimester, how many days remain, how far along the pregnancy is as a percentage, the likely conception window, and a dated table of milestones such as the end of the first trimester, the anatomy scan window, viability, the start of the third trimester, early term, the due date, and the post-term limit. It is meant for expecting parents planning ahead and for anyone who wants to understand how the numbers on a clinic letter were reached.
How it works
The standard estimate, called Naegele's rule, sets the due date 280 days (40 weeks) after the first day of the last menstrual period, on the assumption that ovulation happened on day 14 of a 28-day cycle. For other cycle lengths the calculator adds (cycle length − 28) days, so a 35-day cycle pushes the date a week later. From a known conception or ovulation date it adds 266 days (38 weeks), because pregnancy is dated from a notional period two weeks before conception. For IVF it adds 263 days to a day-3 transfer and 261 days to a day-5 blastocyst transfer, matching the embryo's age. Working backward from a known due date, the equivalent last period is the due date minus 280 days. Gestational age today is the number of days since that equivalent last period, expressed as weeks plus days; the first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days, and the third from 28 weeks 0 days. All dates are handled as local calendar dates so daylight saving changes never shift a result by a day.
Frequently asked questions
How accurate is a due date?
Only about 4 to 5 percent of babies arrive on the estimated date; roughly two-thirds are born within a week of it and most of the rest within two weeks. Full term is anything from 39 weeks 0 days to 40 weeks 6 days, with 37–38 weeks called early term and 41 weeks late term. The date is a planning anchor, not a prediction of the actual day, which is why clinicians prefer to speak of a due month or a due window.
What if I do not know my last period or my cycles are irregular?
Use the conception tab if you know roughly when ovulation happened, or wait for a first-trimester ultrasound: crown-rump length measured between about 7 and 13 weeks dates a pregnancy to within about 5 days and is the most reliable method. When the ultrasound date differs from the period-based date by more than the accepted margin (5 days before 9 weeks, 7 days between 9 and 16 weeks), professional guidelines say the ultrasound date should be used instead.
Why does the calculator count 40 weeks when pregnancy lasts nine months?
Pregnancy is conventionally dated from the first day of the last period, about two weeks before the egg was actually fertilized, so the 40-week count includes two weeks in which the pregnancy had not yet begun. Measured from conception the length is about 38 weeks, or 266 days. Forty weeks is 9 months and about one week on the calendar, so nine months is a fair approximation once you remember where the count starts.
Should I rely on this instead of my doctor's date?
No. This page reproduces the arithmetic that clinics use as a starting point, but your midwife or obstetrician may adjust the date based on ultrasound measurements, your history, or how the pregnancy is progressing, and their date is the one that matters for care decisions. Use this calculator to understand the numbers, plan leave and appointments, or check a rough date before your first visit.